 Measurements of time, we're going to look at the dimensions and the units of that. Now, dimension of time just represents any type of measurement of any physical property that we're talking about the time. It could be a location, it could be a span, it's got the same basic dimension for both of those. And our generic notation for the dimension of time is a capital T, or some textbooks put the square parentheses around there. Now let's take a look at this location and span and see a little bit more about what we mean by these things. For location and time, we're describing when something happens. So that's one of the types of measurements we can make about time. Now, we can talk about this relative to a fixed reference system. And this is what we often refer to when we say when did it happen. We want to know what time, what day, what year, etc. But we can also refer to it relative to the start of an event. So if we're doing with a physics problem, you might say, well, when did it hit the ground? Three seconds after it was dropped. So we can describe that location and time relative to the start of a particular event. Then we get to time spans. How long something lasts? And when we say something, we mean an event. So we could be talking about the class. We could be talking about a particular video and how long does that last? Remember that our time span doesn't tell us exactly when it started and stopped. So a five second time span could be from the start at zero to five seconds. But a five second time span could also be from two seconds to seven seconds. Both of those cases, five seconds elapsed between the start and the end. Now when we're recording all these values, make sure you include units with the values. Particularly with our time spans, we need to know what we're talking about. We've already seen some of these examples as we've looked at things, and people are pretty familiar with the units of times. There's more than the ones I have listed here, but this just gives you a general idea. We want to define our standard unit, though. And for the SI system, the metric system that we're using in physics courses, we refer to the second as our base unit for what we're using. So keep in mind all of these things when we're talking about measurements of time, both its dimensions and its units.